pdm 2.5 breaks tox-pdm

[doloopuntil]

pdm>=2.5 has a new venv management feature, which introduced a breaking change in the semantics of the PDM_USE_VENV environment variable. Running tox with the tox-pdm plugin in a pdm-2.5 setup silently fails to synchronise the dependencies via tox-pdm, which under the hood complains with No virtualenv with key '1' is found, must be one of ['in-project'].

This is because in pdm<2.5 the variable PDM_USE_VENV was a boolean (1 meant using a venv), but in pdm>=2.5 it's the name of the virtual environment to use, so I think this line has to be changed to set PDM_USE_VENV=in-project.

[frostming]

Thanks, the override is not intentional, will be fixed in PDM